Fuel in a fuel tank is evaporated by influcence from the ambient temperature, and its evaporation rate becomes remarkably high especially when the fuel tank is exposed in summer directly to the rays of the sun. If such evaporated fuel is discharged, as it is, into the atmosphere, it becomes one of the causes of air pollution. It follows that the so-called "crankcase storage system" has been adopted, in which the evaporated fuel is stored in the crankcase having a relatively large capacity to prevent the air pollution.
However, even the crankcase has a limited capacity so that it cannot store evaporated fuel any more when it becomes saturated with the fuel.
Therefore, there has been developed a system which has its storage capacity for evaporated fuel increased by connecting the crankcase and the intake passage of the engine so that the evaporated fuel may be sucked out of the crankcase during running operation of the engine until it is fully consumed. With this construction and arrangement, however, if the crankcase reaches saturation while the engine is stopped, the evaporated fuel coming from the crankcase fills up the intake passage so that it not only contaminates the air cleaner but also is sucked from the intake passage into the combustion chamber by the vacuum established in the combustion chamber as the engine is started, whereby the air-fuel ratio (A/F) may become richer than the normal inflammable mixture ratio thereby making it difficult to start the engine.